Add employment application functionality

[stellanl]

This lets an Up user apply to be connected to an organisation without using the Web interface, as discussed in #159

[stellanl]

Just found out that the /rest/v1/user//request_organisation/ entry point requires Django login as well as API key. akvo/akvo-rsr#2074

[stellanl]

One way to improve the UX might be to load organisations and countries as part of the login, if this is the first login. It will take tens of seconds, but it's a one-time occurrence, and the user will have no expectations. The db will then have all necessary info for a connection application; user will not need to do a refresh on that screen.

[stellanl]

Turns out the country must exist in
https://github.com/akvo/akvo-rsr/blob/develop/akvo/codelists/store/codelists_v202.py
and the comparison is case-sensitive, so we cannot use the country code obtained from /rest/v1/country/ without upper-casing it first.

[stellanl]

Application submissions now work, but the approval Email sent to the user could be more informative about Up. (User will need to log out and in again to get access to the new projects.)