* main: move tests to main/tests

Move the templatetags tests to main/tests/test_templatetags.

* main: Add test for templatetags country

Create a test for the templatetag country_flag.

* main: remove duplicate floatvalue

floatvalue is only used in the mirrors templates and the same exact
function exists in the mirror_status templatetags.

* main: Remove duplicate hours filter

The hours filter is also defined in the mirror_status and only used in
mirrors.

* main: move percentage filter to mirrors

Move the percentage filter to the only user of it and add a test for
basic use cases.

* main: remove duplicate duration implementation

The duration templatetag filter is also defined in mirror_status.py

* templates: remove unrequired import flags

* main: Add missing testcase for country_flag

Add the None test case, so that the function is fully covered.

* todolists: create tests for Todolist model

Add basic tests for the Todolist model