Add manual share links to answerboxes

[pixeldesu]

This is mainly there to "circumvent" the issue with automated sharing, offering links to the share widgets of Twitter and Tumblr.

Also moves the general text preparation into own SocialHelper:: modules, so that it can be reused without duplicating code. Also added tests for all new things!

Also fixes the bug with the OS share menus which only worked on the initial set of answers, that now works on all of them.

[codecov]

Codecov Report

Merging #232 (453724b) into master (2d776e7) will increase coverage by 0.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
+ Coverage   61.08%   61.64%   +0.55%     
==========================================
  Files          89       92       +3     
  Lines        2092     2109      +17     
==========================================
+ Hits         1278     1300      +22     
+ Misses        814      809       -5     

Impacted Files	Coverage Δ
app/helpers/social_helper.rb	100.00% <100.00%> (ø)
app/helpers/social_helper/tumblr_methods.rb	100.00% <100.00%> (ø)
app/helpers/social_helper/twitter_methods.rb	100.00% <100.00%> (ø)
app/models/services/tumblr.rb	58.33% <100.00%> (+13.88%)	⬆️
app/models/services/twitter.rb	92.30% <100.00%> (-4.13%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2d776e7...453724b. Read the comment docs.