Cleanup: Remove upgrade code that is no longer supported

In 3bac7afb we dropped the ability to upgrade directly from 2.1.0 or earlier,
so this code is never being run and thus is no longer necessary.
Cleanup: Only import if necessary
Cleanup: Ensure that save_version_function is actually a function

This also improves readability.
Cleanup: Import just what we need
Cleanup: Fix import order
Cleanup: It is not necessary to check for build absence

It is always passed.
Cleanup: Use the right prefixes to avoid checks
Cleanup: Avoid unnecessary conversions
Cleanup: Move import to top of the function
Cleanup: Import just what we need
Cleanup: Rename constants to improve readability
Cleanup: Just save the build versions

There is no need to check for keys existence because we are initializing a
blank database.
Cleanup: Move code to separate function on initdb
Cleanup: Simplify code by merging functions
Cleanup: Remove intermediate function to save legacy build version
Cleanup: Remove unnecessary Meta from form

It appears to be used on djblets templates that are not used.
Partial pick of Evernote's evernote/pootle@2f453d22

Pick note: These changes might be required by the following picks.

    The rest of the changes in that commit are either necessary or where
    later dropped or changed at Evernote.
Make #footer stick to bottom

Conflicts:
	pootle/templates/layout.html
URL cleanups in Evernote dropped code

Change last URL template tags using references to views to instead use URL
names.
Make #footer stick to bottom