Interpolate variables in redirection

[groenroos]

Standardises the various redirection points into a single Redirect lib, and implement variable interpolation as per #299.

Additionally, fixes some query parameter related stuff.

Closes #299.

[codecov]

Codecov Report

Merging #322 (f04f141) into master (67515b9) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
+ Coverage   98.21%   98.24%   +0.03%     
==========================================
  Files          36       37       +1     
  Lines        4416     4450      +34     
==========================================
+ Hits         4337     4372      +35     
+ Misses         79       78       -1     

Impacted Files	Coverage Δ
hooks/sapling/user/forgot.js	100.00% <100.00%> (ø)
hooks/sapling/user/login.js	100.00% <100.00%> (ø)
hooks/sapling/user/logout.js	100.00% <100.00%> (ø)
hooks/sapling/user/recover.js	100.00% <100.00%> (ø)
hooks/sapling/user/register.js	98.09% <100.00%> (-0.02%)	⬇️
hooks/sapling/user/update.js	100.00% <100.00%> (ø)
lib/Redirect.js	100.00% <100.00%> (ø)
lib/Request.js	99.26% <100.00%> (+0.33%)	⬆️
lib/Response.js	98.50% <100.00%> (+<0.01%)	⬆️

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