Replace AccountPage with a new login flow

The old login flow has a couple of issues:

• The login page is technically duplicated on the settings page, and also in AccountPage
• AccountPage served two purposes, one is being a login page but also account management (which is currently really simple, because we can only log out)
• When logging in, only a passive notification appears which is easy to miss and it doesn't return you to where you were.
• It doesn't use the standard mobile form layout like settings.

Now everything is standardized into a new login flow, in LoginPage.qml!

It has a familiar mobile form layout, complete with validation:

(Note that the passive notification is still shown for login errors, because I didn't want to change the API here right now)

Account management has been simplified, and done directly from the settings page, under the accounts section:

(I forgot to add a separator, so that's fixed but not reflected in this screenshot)

When logged in, the page is also automatically popped too. I also made it so it takes the instance URL you already have configured, instead of from a preselected list which doesn't make sense - you usually want to log in using the same instance you use to view. I haven't removed the instance URL entirely off this page in case that was intentional.