Add automatic coloring of tags

Please see this as proposal on how to implement automatic/random colors
for tags while keeping the current set of hard coded colors in place (at
least in the frontend).

Bear with me as I have even less Angular knowledge than Django and just
tried to get away with as few changes as possible. :-) AIUI you want to
change to a color picking system anyways in the future, which could also
play well with this.

fixes #51

Pipfile

@@ -40,6 +40,7 @@ whoosh="~=2.7.4"
 inotifyrecursive = ">=0.3.4"
 ocrmypdf = "*"
 tqdm = "*"
+colorhash = "*"
 
 [dev-packages]
 coveralls = "*"