Add msgctxt support for proper context in pot files

Our pot files should use the msgctxt field to store context rather than prefixing the message. This will allow context to be rendered correctly in Crowdin.

Crowdin has a field beneath the string in the translation view where context is shown. It is always empty for GitLab because we don't use msgctxt

Reference: http://pology.nedohodnik.net/doc/user/en_US/ch-poformat.html#sec-podisamb