Fix #2834: Merge the change events 'delete' and 'add' as 'update' for Activity Feed

[vivekratnavel]

Describe your changes :

See #2834

In this patch, I have implemented the following changes:

• Added support for column level entity links
• Merged add and delete events into one update event
• Show inline diff of the changes done (with html tags for UI to render, since there is no markdown support for font colors)
• Add change type to the thread's message to know whether its a minor or a major version change
• Added unit tests

Type of change :

• Bug fix
• Improvement
• New feature
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation

Frontend Preview (Screenshots) :

The API response of Feed API will now be like this with html tags that can rendered nicely in the UI:

{
           "id": "5c0e6146-bfbd-446d-a335-a3e9d21fb852",
           "href": "http://localhost:8585/api/v1/feed/5c0e6146-bfbd-446d-a335-a3e9d21fb852",
           "threadTs": 1645943657593,
           "about": "<#E/table/local_mysql.sample_db.lineorder/columns>",
           "createdBy": "anonymous",
           "updatedAt": 1645943657593,
           "updatedBy": "anonymous",
           "resolved": false,
           "message": "Updated columns.lo_orderpriority : <br/> dataType: `<span class=\"diff-removed\">\"BLOB\"</span><span class=\"diff-added\">\"INT\"</span>` <br/> dataTypeDisplay: `<span class=\"diff-removed\">\"blob\"</span><span class=\"diff-added\">\"int\"</span>` <br/><br/> **Change Type:** *MAJOR (7.2 -> 8.2)*",
           "postsCount": 0,
           "posts": []
       },

Checklist:

• I have read the CONTRIBUTING document.
• I have performed a self-review of my own.
• I have tagged my reviewers below.
• I have commented on my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• All new and existing tests passed.

[vivekratnavel]

The failing tests are not related to this change.

2022-02-27T10:08:44.8205752Z [ERROR] Errors: 
2022-02-27T10:08:44.8269348Z [ERROR]   RoleResourceTest>EntityResourceTest.get_entityWithDifferentFieldsQueryParam:486->EntityResourceTest.listEntities:1900->EntityResourceTest.listEntities:1914 » HttpResponse