[#55466150] Add an update action to WikisController.
[#55466150] Show the wiki logo in the header.
[#55466150] Assign a bunch of things in `update/create` actions that were assigned in `edit/new`
[#55466150] Add a default wiki logo.
[#55466150] Don't allow logos larger than 100px in width.
[#55466150] Reword the logo hint text (in the Settings)