Upgrading the asset pipeline gave me a chance to tidy up some JavaScript and fix some old issues.

Changes

Use propshaft instead of sprockets for css and asset handling
Use jsbundling with webpack, rather than the deprecated webpacker gem, for JavaScript managing. -> Closes #71
- Reenable ujs so confirmations work again and we can use links for everything again.

Fun JavaScript things

Replace tag selection with a multiple select2 field, rather than lots of tickboxes. Makes the forms more compact, and no functionality is lost. -> Fixes #251
Select2 field for user field now uses pagination with search, rather than loading all users. -> Implements #247
Validate all input fields at page load and on change, except when explicitly marked as not needing validation or when they are part of a search form. -> Fixes #246
Convert the final two coffeescripts, that are unused anyway. -> Fixes #161
Replace the old alert script that was
Rewrite some scripts to vanilla js, take out inline scripts to their ownfiles, and generally comment scripts now I understand what they all do.
Add dedicated JavaScript for showing flash messages.

Fix Markdown Editor previews -> Fixes #260
Fix calling the md_editor after insert function when dynamically adding nested fields with cocoon -> Fixes #253