5.0 prerelease 65

This is a first official prerelease for Thunderbird 78. Still has some minor issues but the core functionality is there. Once we decide to upmerge we will create a new branch for 68 to service users of old versions.

Known issues:

• drag over doesn't look correct yet, there is no visual feedback when dragging mail or folders / tabs over existing tabs. This can make it difficult to reorder tabs as the insert position is also not shown.