Central Server deprecation

[benjaoming]

Fixes #327 #328 #329

1. [x] Omnipresent notification text, please add suggestions in this PR
2. [x] Removal of new signups (only One-Click Registration allowed)

CC: @carinediaz @jamalex

[codecov-io]

Codecov Report

Merging #330 into develop will decrease coverage by 0.01%. The diff coverage is 50%.

@@             Coverage Diff             @@
##           develop     #330      +/-   ##
===========================================
- Coverage    74.06%   74.04%   -0.02%     
===========================================
  Files           60       60              
  Lines         2445     2447       +2     
===========================================
+ Hits          1811     1812       +1     
- Misses         634      635       +1

Impacted Files	Coverage Δ
centralserver/registration/views.py	78.63% <50%> (-0.5%)	:arrow_down:

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[benjaoming]

Here is the flow of the registrations being disabled. Note that step 1 cannot be changed and step 2 is unchanged from previously - I thought it's better to dedicate a separate page for the "Registration closed" message.

1) In KA Lite, people click this button after a fresh installation...

2) Now, they end up at this page and should click "sign up here":

3) After which they are told that registration is closed.

[benjaoming]

I am fully aware that all the lines are to long to be reader-friendly, but I'm not going to fix the Stylesheets on the central server. All the layouts are somewhat doubtful in hindsight.

[benjaoming]

Main landing page on kalite.learningequality.org