Re-license under MIT all work from 2021-04-07 onwards

This is allowed because since 2021-04-06 (when PR
#11777 was merged), our pull
request template contains a checkbox asking the contributor to allow the
Galaxy committers to license all their contributions to the core galaxy
codebase under the MIT license.

Explicit approval to this change will be sought from the few persons who:
- opened a PR before 2021-04-07 which was merged after this date, and
- haven't already agreed to the re-licensing in a later PR.

.github/pull_request_template.md

@@ -10,5 +10,4 @@
   1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]
 
 ## License
-- [x] I agree to license these contributions under [Galaxy's current license](https://github.com/galaxyproject/galaxy/blob/dev/LICENSE.txt).
-- [x] I agree to allow the Galaxy committers to license these and all my past contributions to the core galaxy codebase under the [MIT license](https://opensource.org/licenses/MIT). If this condition is an issue, uncheck and just let us know why with an e-mail to [email protected].
+- [x] I agree to license these and all my past contributions to the core galaxy codebase under the [MIT license](https://opensource.org/licenses/MIT).

LICENSE.txt

@@ -1,4 +1,57 @@
-Copyright (c) 2005-2016 Galaxy Contributors (see CONTRIBUTORS.md)
+Copyright (c) 2005-2022 Galaxy Contributors (see CONTRIBUTORS.md)
+
+Work contributed from 2021-04-07 onwards is licensed under the MIT License.
+Work contributed before this date is licensed under the Academic Free License
+version 3.0.
+See https://github.com/galaxyproject/galaxy/ for the contribution history.
+See below for the full text of both licenses.
+
+
+Some icons found in Galaxy are from the Silk Icons set, available under
+the Creative Commons Attribution 2.5 License, from:
+
+http://www.famfamfam.com/lab/icons/silk/
+
+
+Other images and documentation are licensed under the Creative Commons
+Attribution 3.0 (CC BY 3.0) License. See:
+
+http://creativecommons.org/licenses/by/3.0/
+
+
+--------------------------------------------------------------------------------
+
+
+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+
+--------------------------------------------------------------------------------
+
+
+Academic Free License ("AFL") v. 3.0
+
+This Academic Free License (the "License") applies to any original work of
+authorship (the "Original Work") whose owner (the "Licensor") has placed the
+following licensing notice adjacent to the copyright notice for the Original
+Work:
 
 Licensed under the Academic Free License version 3.0
 
@@ -173,14 +226,3 @@ Licensed under the Academic Free License version 3.0
     source software unless your Modified License has been approved by Open 
     Source Initiative (OSI) and You comply with its license review and 
     certification process.
-
-
-Some icons found in Galaxy are from the Silk Icons set, available under
-the Creative Commons Attribution 2.5 License, from:
-
-http://www.famfamfam.com/lab/icons/silk/
-
-
-Other images and documentation are licensed under the Creative Commons Attribution 3.0 (CC BY 3.0) License.   See 
-
-http://creativecommons.org/licenses/by/3.0/