Refactor routes code, save form when back from add location

[wbazant]

Closes #604
Closes #601
Closes #652

The code would now support a "back button saves edits" for edit location / add review / edit review, and originally I saw myself adding those, but then realised they're not really useful features.

[ezwelty]

@wbazant No issues found with #601 and #604. Regarding #652:

• Sentence case please (it is de rigeur in the existing translations). Also these text snippets (except for "Editing Review") already exist in the mobile app locale files:

  "menu.add_location": "Add location",
  "menu.add_review": "Add review",
  "menu.edit_location": "Edit location",

https://github.com/falling-fruit/falling-fruit-mobile/blob/f29322747bc0bbfc026563c68b8bf26f5c976070/www/locales/en.json#L38

• I'm not super fond of /locations/:locationId/edit/details because it diverges from the existing website. Can we use /locations/:locationId/edit for the form (form + map on desktop) and /locations/:locationId/edit/position for the position (mobile only), or does this compromise the new website route-logic somehow?

[wbazant]

Thanks! I agree with you /locations/:locationId/edit is a better path - I'm not even sure why I standardised them this way instead of the other way, maybe having .../details and .../position was a bit simpler for writing the routes, but that's not a user benefit.

I'll merge this and I've made an issue #672 to use the verbiage from the mobile upp for the headers - it needs a bit of unrelated work, so I'll do it on a new branch as part of the translation epic.