Page now also handles failing api requests when searching.

Resolves: GREG-134

All A and AA problems have been fixed except a missing label on an input
field.

Serialization has been moved to serializer classes, sharing them when
sensible and Views have been converted to ViewSets to simplify code.

Allows search using e-mail and mobile phone, limiting results to 10
first matches

When choosing the Register new guest button, you are now sent to a new
page where you can search for existing guests to prevent creating
duplicates. The page also has a register new guest button that takes you
to the previous version of the page, now located at /register/new

A future commit will make the search field functional.