List filtering (Fixes #32)

[neka-io]

Includes backend code for filtering based on principal title, time range and event name. Filtering based on country, region and event_type complicated by the fact that they are foreign keys on the Event model, which in turn has a many-to-many relationship with Trip.

To be completed:

• [x] Backend code for country, region and event_type filtering
• [x] Frontend code for all filtering functions
  • [x] Include calendar widget that translates Month or Year selections to date ranges

[codecov-io]

Current coverage is 37.85%

Merging #156 into develop will decrease coverage by -4.17% as of 96fc139

@@            develop    #156   diff @@
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  Files             7       9     +2
  Stmts           345     383    +38
  Branches          0       0       
  Methods           0       0       
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  Hit             145     145       
  Partial           0       0       
- Missed          200     238    +38

Review entire Coverage Diff as of 96fc139

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