[#139] Add colors to categories

Add colors to categories chips.

• Visible in gallery filter, app and remote app details, app edit
• Colors are persisted in database, in category model (CharField(max_length=8))

Don't forget to migrate with:

./manage.py migrate

• Create a new migration instead of editing an old one
• Update colors from https://gitlab.com/fdroid/fdroidclient/blob/master/app/src/main/res/values/colors.xml#L9-25
• Category colors in gallery screen repositories added by endless scroll