Enabled asynchronous learning from user using RQ

[tonioo]

see #160

[codecov]

Codecov Report

Merging #162 (ac929a6) into master (de13b12) will increase coverage by 0.06%.
Report is 2 commits behind head on master.
The diff coverage is 92.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
+ Coverage   91.57%   91.63%   +0.06%     
==========================================
  Files          22       23       +1     
  Lines        1377     1387      +10     
==========================================
+ Hits         1261     1271      +10     
  Misses        116      116              

Files	Coverage Δ
modoboa_amavis/views.py	86.95% <100.00%> (+0.26%)	⬆️
modoboa_amavis/tasks.py	90.00% <90.00%> (ø)

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