[WIP] Opportunities detail styles

[gemfarmer]

⚠️ WIP until tests have been added for templatetags.

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Addresses the wireframe in Trello ticket Build out Wireframes for Detail Opportunity View

Specifically this PR

• Creates a template for details page data
• Makes details page responsive
• Creates/styles a responsive site-wide header and footer
• Adds font awesome font library for use of icons on the page (we can replace with svgs or a different font icon library if preferred)
• Adds a django custom filter script

Looks more or less like this:

[codecov-io]

Current coverage is 90.86%

Merging #52 into master will increase coverage by +0.31% as of 4180d5c

@@            master     #52   diff @@
======================================
  Files            9      10     +1
  Stmts          360     372    +12
  Branches        17      17       
  Methods          0       0       
======================================
+ Hit            326     338    +12
  Partial          3       3       
  Missed          31      31       

Review entire Coverage Diff as of 4180d5c

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[stvnrlly]

This is all merged separately as part of #58.