Add MathJax to Django Admin

[mimischi]

We now have a live preview for the chemical_formula field in the Django admin using MathJax and VueJS.

Changelog

• The chemical_formula field now has a live preview, when editing MineralType objects. We're using VueJS here.
• The initial code was found on JSFiddle (http://jsfiddle.net/akiyah/vevygfzb/) and was originally written by user akiyah.
• We use very lazily written CSS.
• Fixes #12.

[codecov-io]

Codecov Report

Merging #218 into develop will increase coverage by 0.05%. The diff coverage is 88.88%.

@@             Coverage Diff             @@
##           develop     #218      +/-   ##
===========================================
+ Coverage    80.99%   81.04%   +0.05%     
===========================================
  Files           12       12              
  Lines          505      517      +12     
===========================================
+ Hits           409      419      +10     
- Misses          96       98       +2

Impacted Files	Coverage Δ
geomat/stein/admin.py	81.96% <100%> (+0.3%)	:arrow_up:
geomat/stein/forms.py	89.47% <84.61%> (-10.53%)	:arrow_down:

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