Revert to single comma for description barrier

Created by: GeraldJansen

This PR reverts the use of a double comma to indicate the start of the description, a breaking change introduced in Hamster 3.0, back to the previous use of a single comma, as discussed in #657. Likewise, the double comma needed before tags, in the case of descriptions containing the #hash pattern, is also reverted to the use of a single comma.

This change requires two restrictions to the parsing rules. Firstly, no comma is allowed in the activity name (#280 (closed)). Secondly, tags may not be separated by a comma when entered on the commandline (ie. just use #tag1 #tag2, not #tag1, #tag2). The differences in tests/test_stuff.py illustrate the changes in behaviour of the parser.