overrides: Fix tagBuild call

koji.tagBuild does not take a strict kwarg.
overrides: Properly validate the expiration_date

If the user didn't specify it, this validator would just fail with a
KeyError.

Instead, it needs to just not do anything, and let colander
automatically report the error that a required parameter wasn't
specified.
overrides: Remove duplicate check
overrides: Fix getting the object
overrides: Don't allow editing the build

Editing the build an override is in just makes things very complex,
for not much gain.
overrides: Fix a unit test

Builds need a release.
overrides: Fix build validator

It makes no sense to require that the build already exists in the DB.

If it doesn't, we just need to create it.

However in both cases, we should ensure it is properly tagged.
tests: Improve override creation test
overrides: Automatically untag older overrides

If there is already a buildroot override for build 1 of package foo in
release bar, then creating a new buildroot override for a new build of
the same package in the same release should expire the previous one
automatically.
overrides: Don't expire an already expired override
overrides: Unexpire when expiration_date is set in the future
overrides: Submit new buildroot overrides via the web UI
overrides: List existing buildroot overrides
user: Display latest overrides on the profile page
overrides: Display a buildroot override

This also allows editing it in the web UI.
overrides: Tell the user how to know whether it is active
overrides: Explicitly specify all edition parameters
js: Bundle the Bootstrap date picker

This will be useful for a better buildroot override form.
overrides: Use the date picker widget

This makes the form much more pleasant to use.
user: Reuse the updates/overrides tables

This allows deleting lots of lines of code.

At the same time, it actually improves the list of recent updates on the
user page: it was missing the popup with the actual date when mousing
over the age.