Make email templates translatable

[ThiefMaster]

Email templates are the one last thing in Indico that's basically untranslated; the templates have no i18n markers and are only in english, but some users require them to be translated as well, and as e.g. #5062 indicates this is a major hassle right now.

Main tasks here:

• Add translation markers to email templates; be especially careful in case of plaintext emails to not break the formatting. Ideally, a unit test with a snapshot is added for each of these email templates (see test_move_request_creation_email_plaintext for an example) before touching them to ensure the formatting/spacing does not change.
• Provide some way (e.g. a contextmanager) to temporarily switch the active language, either to a specific language or to that of a given user.
• Use this whenever we send emails to use the recipient's language if there is one or (in case the recipient is not an Indico user) a reasonable default (DEFAULT_LOCALE)

AFAIK registration emails currently interpolate a title-cased enum key name somewhere, iirc. for the state. This probably needs to be fixed to use a properly translated string, but it'd be OK to open a separate issue for it since it's not directly part of making emails translatable.

[tomasr8]

For your second point, we have session_language which seems to work nicely with get_template_module. I'm curious what we should do when there are multiple recipients, each with a potentially different language like in the case of the abstract comments or paper comments. Worth it to send the emails separately or just use DEFAULT_LOCALE in that case?