login_flow.ui: Add mnemonics for keyboard navigation

mnemonics are a tool used for faster keyboard navigation,
and help with accessibility.

This commit adds mnemonics for every entry and button that
either has a label or doesn't have another shortcut.
4 jobs for <span class="ref-name">wip/christopherdavis/redesign-login-flow</span> in 25 minutes and 7 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Check
passed #273229
check

00:00:35

 
  Test
passed #273230
flatpak

00:20:41

 
  Review
passed #273231
review

00:02:56

failed #273232
allowed to fail manual
stop_review

00:00:54

 
Name Stage Failure
failed
stop_review Review
Created fresh repository.
From https://gitlab.gnome.org/BrainBlasted/fractal
* [new branch] master -> origin/master
* [new branch] room_member_events -> origin/room_member_events
* [new branch] wip/christopherdavis/fit-room-directory -> origin/wip/christopherdavis/fit-room-directory
* [new tag] 0.1.0 -> 0.1.0
Checking out e1fbb54c as wip/christopherdavis/redesign-login-flow...
fatal: reference is not a tree: e1fbb54ca448aa6342fd2974e8361b6dde2aaa06
ERROR: Job failed: exit code 1