[WIP] add masker component and leaflet dependency

[aeschylus]

Getting a better start on this through the edit pages. Still figuring out which options will allow deletion and repositioning of masks in leaflet. Playing around with options here: http://blockbuilder.org/anonymous/c55380eeb3523ba1395918bc9ba97aaf

The edit remove option doesn't seem to behave as advertised, as the delete icon is always grayed out.

Still a lot to go on this, but more straightforward as part of the edit step:

• [x] wrap masker component in the base leaflet style component
• [x] edit options to allow repositioning and deletion of masks
• [x] add stylesheet
• [x] add mask polygon serialisation to form submission
• [x] handle polygon serialisation in controller action
• [x] update model and schema to restore existing masks
• [x] detect and parse existing masks to restore for editing
• [ ] handle null-case for masks through defaultProps
• [ ] tests

[codecov-io]

Codecov Report

Merging #118 into master will increase coverage by -3.05%.

@@            Coverage Diff            @@
##           master    #118      +/-   ##
=========================================
- Coverage   98.84%   95.8%   -3.05%     
=========================================
  Files          31      31              
  Lines         260     262       +2     
=========================================
- Hits          257     251       -6     
- Misses          3      11       +8

Impacted Files	Coverage Δ
app/controllers/image_templates_controller.rb	56.25% <ø> (-43.75%)	:x:
...ntrollers/collection_templates/build_controller.rb	97.14% <ø> (-2.86%)	:x:
app/models/image_template.rb	100% <100%> (ø)	:white_check_mark:

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