* Old legal terms are stored using git at compile time, with their diffs to
the most recent version.
* User's consent is now asked explicitly. If user has consented to old
version of legal terms, they will be asked for renewed consent and shown
the diff. They cannot use any page before that.
* Legal terms were added to signup form for reading.
* Underage users cannot use the site, so legal age is asked at signup and
at consent renewal.
* Refactored code for betterness.
Closes #58, #63