Add support for frozen release state

[mattiaverga]

The frozen release state has been available for a while, but it was never used in releng workflow. There are also many bits missing from Bodhi code to be functional. This PR will add those bits.

• display frozen releases in release list (fixes Release freeze is not clear at all #4103 )
• show an info pane on updates pages informing user that the release is frozen
• allow bodhi CLI to edit a release state into frozen state
• make bodhi-push work with frozen releases
• force updates of frozen releases into testing before requesting them to stable

About bodhi-push, it now defaults to push stable composes for pending + current releases (just like before) and push testing composes for pending + current + frozen. This will avoid releng to stop the automatic cronjob in freeze periods (fixes #4478 ). It allows to push to stable specific updates or builds even if the release is frozen, though.

[lgtm-com]

This pull request introduces 1 alert when merging 3212c7d into 85e6c48 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 1 alert when merging 7e73d04 into 85e6c48 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

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