Some small improvements to events

[ddabble]

Proposed changes

Improvements

• Prevent registering for an event without any (past or future) occurrences (665d86f945e027b07a933d7ed09071b2735c27f9)
• Don't show the "is old" message when an event has no occurrences (371cd7f8ddef46cc6549086889fe2a8d35dcac37)
• Started using new label names for each event type, which should be clearer with regard to their intended usage. Also made their description - shown when clicking / hovering over the yellow question marks in an event form - (hopefully) easier to understand (e44802c795f68022c4dc753cf89bac3a0371bfb2)

Areas to review closely

Investigate the possibility of further improving the event type descriptions, to make them easier to understand for users.

Checklist

(If any of the points are not relevant, mark them as checked)

• [x] Created tests that fail without the changes, if relevant/possible
• [x] Made sure that your code conforms to the code style guides and that any common code smells have been addressed
  • (It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you keep it in mind while taking a second look at your code before opening a pull request)
• [x] Added sufficient documentation - e.g. as docstrings or in the README, if suitable
• [x] Added your changes to the "Unreleased" section of the changelog - mainly the changes that are of particular interest to users and/or developers, if any
• [x] Added a "Deployment notes" section above, if anything out of the ordinary should be done when deploying these changes to the server

[codecov[bot]]

Codecov Report

Merging #571 (b78dea9) into dev (cb33517) will increase coverage by 0.72%. The diff coverage is 66.66%.

:exclamation: Current head b78dea9 differs from pull request most recent head e573840. Consider uploading reports for the commit e573840 to get more accurate results

@@            Coverage Diff             @@
##              dev     #571      +/-   ##
==========================================
+ Coverage   87.71%   88.43%   +0.72%     
==========================================
  Files         134      132       -2     
  Lines        5585     5519      -66     
==========================================
- Hits         4899     4881      -18     
+ Misses        686      638      -48     

Impacted Files	Coverage Δ
news/models.py	98.06% <50.00%> (-0.63%)	:arrow_down:
news/views.py	90.26% <75.00%> (-0.20%)	:arrow_down:
checkin/models.py	91.07% <0.00%> (-1.53%)	:arrow_down:
util/url_utils.py	91.30% <0.00%> (-1.29%)	:arrow_down:
internal/forms.py	86.91% <0.00%> (-0.25%)	:arrow_down:
web/widgets.py	95.07% <0.00%> (-0.11%)	:arrow_down:
make_queue/views/api/calendar.py	97.61% <0.00%> (-0.06%)	:arrow_down:
util/view_utils.py	96.73% <0.00%> (-0.04%)	:arrow_down:
util/templatetags/datetime_tags.py	98.36% <0.00%> (-0.03%)	:arrow_down:
... and 24 more

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