user-accounts: Use GtkTemplate

Switch from GtkBuilder to using GtkTemplate.
Rename .[ch] and .ui files to standard names.
Rename widget IDs to be more readable.
Drop widget IDs that are not used.
Move code into the .ui file that can be.
Connect signals in swapped form.
11 jobs for users-gtktemplate in 5 minutes and 31 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #132508
build

00:02:07

 
  Test
passed #132510
asan

00:03:23

passed #132509
test

00:00:51

 
  Manual
manual #132515
allowed to fail manual
armv8
manual #132511
allowed to fail manual
flatpak
manual #132514
allowed to fail manual
msan
manual #132516
allowed to fail manual
ppc64le
manual #132512
allowed to fail manual
tsan
manual #132513
allowed to fail manual
ubsan
 
  Review
manual #132517
allowed to fail manual
review
manual #132518
allowed to fail manual
stop_review