Resolve "Remove hard dependency on Chronos"

Closes #14

See merge request !10

Created a row of issues on the way 😜

Resolve "Fix REBUS title"

Closes #7

See merge request !9

Currently translated at 1.7% (1 of 58 strings)

Translation: AlekSIS/AlekSIS-App-Hjelp
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-hjelp/la/

Currently translated at 39.6% (23 of 58 strings)

Translation: AlekSIS/AlekSIS-App-Hjelp
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-hjelp/de/

Currently translated at 39.6% (23 of 58 strings)

Translation: AlekSIS/AlekSIS-App-Hjelp
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-hjelp/de/

Resolve "Rewrite REBUS with django-select2"

Closes #6

See merge request !8

Merge remote-tracking branch 'origin/6-rewrite-rebus-with-django-select2' into 6-rewrite-rebus-with-django-select2

Change rebus.html accordingly.
Fixed typo in migration.

Remove materialize-select2.js reference since it is not really needed in order to achieve a material design look for select2.

Completely changes the method used to display a simple TextInput field in the dynamic bug_categories row; now using an extra CharField which is turned visible/invisible.
Add free_text field to models.py, change migrations and admin.py accordingly.
Added modified top/bottom margins in hjelp.css.
Further disables all fields that are not visible and simplifies jQuery section of rebus.html.
Modifies models.py so that children are deleted when the free text option is selected.

rebus.html template: merge the ability to show/hide the next select field into setNextProperties function, automatically does not display next field if there are no children present

- Models: remove toplevel field and replace it with placeholder field for children, adding tagging option for children
- Migrations: change 0002_add_bug_report.py according to models.py
- Forms: add default placeholder for first field, remove queryset filtering according to toplevel attribute
- Admin: remove displaying of toplevel and replacing it by placeholder, add tagging
- Views/URLs: replace rebus_get_icon AJAX request with a new rebus_get_next_properties which also includes the children's placeholder and tagging possibility
- rebus.html template: remove default bug category labels, change javascript updating of next field according to new AJAX request, add functionality to toggle the ability to use tagging input and to change placeholder dynamically

add 0003_alter_faqquestion_answer_text_field.py in order to change faqquestion answer_text field type

- Optimize ajax requests and JS
- Optimize hiding of bug category fields