Neilj/fix autojoin

[neilisfragile]

In the case where the server requires consent, then auto joining cannot happen until consent has been granted by the user - this PR calls the auto join code after consent has been granted and calls it immediately if consent is not required

[codecov-io]

Codecov Report

Merging #4223 into develop will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #4223      +/-   ##
===========================================
+ Coverage    73.52%   73.55%   +0.03%     
===========================================
  Files          299      299              
  Lines        29798    29806       +8     
  Branches      4871     4872       +1     
===========================================
+ Hits         21908    21923      +15     
+ Misses        6465     6454      -11     
- Partials      1425     1429       +4

Impacted Files	Coverage Δ
synapse/rest/consent/consent_resource.py	74.48% <100%> (+0.53%)	⬆️
synapse/rest/client/v2_alpha/register.py	60.22% <100%> (+0.14%)	⬆️
synapse/handlers/register.py	70.74% <100%> (+0.65%)	⬆️
synapse/handlers/search.py	81.25% <0%> (ø)	⬆️
synapse/handlers/federation.py	61.72% <0%> (ø)	⬆️
synapse/state/v1.py	92.24% <0%> (+1.55%)	⬆️
synapse/handlers/message.py	83.5% <0%> (+1.71%)	⬆️

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