refactor!: make automatically following documents optional

[hasnain2808]

Creating content for Document Follow emails is very heavy.
It collects all changes and comments for each document followed by a user

Changes:

1. Documents will not be followed automatically but there is an option to turn this behavior on (Backwards Compatibility).

2. Minor optimizations on how the users and changes are collected

3. Limiting document per user to 50. Ordered by the latest document followed. This will make the job execution time to depend only on the number of users and the number of documents they are following

Typical Document Follow Email for reference

Docs for Document Follow: https://docs.erpnext.com/docs/v13/user/manual/en/setting-up/email/document-follow

[codecov]

Codecov Report

Merging #16030 (a8dc432) into develop (833eeff) will decrease coverage by 1.55%.
The diff coverage is 76.08%.

@@             Coverage Diff             @@
##           develop   #16030      +/-   ##
===========================================
- Coverage    56.80%   55.25%   -1.56%     
===========================================
  Files          758      756       -2     
  Lines        67423    67261     -162     
  Branches      5797     5770      -27     
===========================================
- Hits         38297    37162    -1135     
- Misses       25576    26275     +699     
- Partials      3550     3824     +274     

Flag	Coverage Δ
server	62.07% <76.08%> (-0.22%)	⬇️
ui-tests	42.97% <ø> (-4.04%)	⬇️

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.

[hasnain2808]

Test Failures caused by: #15964