Fix matching logic

[SaptakS]

Description

• This PR ensures that all the matchings use the same matching logic (i.e. the keyword property described in model which checks for a keyword in keywords, issue title, issue description).
• While fixing this, I also noticed that all the keywords were not being saved, so fixed that too.
• Currently, in explorer when we search (also from onboarding) it wasn't checking for all the keywords in the comma separated string. Instead it was either checking the first keyword or the entire string thus not always giving good result. So modified queryset query to give an union of all the queries with the different keyword.

For simplicity of review, I have made separate commits for each.

Checklist

• Read and conforms to the contributor guidelines
• Read and conforms to the style guidelines
• Linter status: 100% pass
• Added relevant tests
• Tests pass and code coverage has not decreased
• Changes don't break existing behavior
• Commit message follows commit guidelines
• Code documentation
• Completed: Code review by core team and any requested changes

Affected core subsystem(s)

Search and matching

Refers/Fixes

Fixes #3990
Fixes #4048

[codecov]

Codecov Report

Merging #4050 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4050      +/-   ##
==========================================
- Coverage   30.22%   30.19%   -0.03%     
==========================================
  Files         205      205              
  Lines       16240    16234       -6     
  Branches     2139     2136       -3     
==========================================
- Hits         4908     4902       -6     
  Misses      11158    11158              
  Partials      174      174

Impacted Files	Coverage Δ
app/dashboard/router.py	30.25% <0%> (-0.64%)	⬇️
...arketing/management/commands/new_bounties_email.py	0% <0%> (ø)	⬆️
app/dashboard/embed.py	28.16% <0%> (-3.45%)	⬇️

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[danlipert]

Nice! Tested locally and works great!