Flash app messages to user; users can only upload pictures

[alysivji]

Closes #303

What does this do

We were using jsonify to pass information back to the user. This was a hack until we figured out how to add messages to our flask templates.

Restrict the file types users can upload to .jpg, .jpeg, .png

Why are we doing this

Improve user experience

How should this be tested

Tested locally to see if messages come thru; it's good

Migrations

n/a

Dependencies

n/a

Callouts

n/a

[codecov-commenter]

Codecov Report

Merging #322 into master will decrease coverage by 0.05%.
The diff coverage is 65.21%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
- Coverage   92.48%   92.42%   -0.06%     
==========================================
  Files          71       71              
  Lines        2394     2403       +9     
==========================================
+ Hits         2214     2221       +7     
- Misses        180      182       +2     

Impacted Files	Coverage Δ
busy_beaver/apps/web/views.py	77.59% <57.89%> (-0.19%)	⬇️
busy_beaver/apps/web/forms.py	100.00% <100.00%> (ø)

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