feat: restricts unverified user for buying free tickets

[shreyanshdwivedi]

Related to fossasia/open-event-attendee-android#1881 (comment)

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

Changes proposed in this pull request:

It adds a check to restrict an unverified user from buying free tickets

[kushthedude]

@shreyanshdwivedi Are you sure all mentors have approved it because this will break the flow for Guest Order Form ?

[shreyanshdwivedi]

@kushthedude please refer to the issue mentioned and @iamareebjamal asked me to add this check on server later in that issue itself.
Also, I've checked for guest orders, it'll only stop them from buying free tickets. Paid tickets can be bought. Please check
Edit: I'll fix travis till then

[codecov]

Codecov Report

Merging #6008 into development will decrease coverage by 0.03%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           development    #6008      +/-   ##
===============================================
- Coverage        66.29%   66.25%   -0.04%     
===============================================
  Files              285      285              
  Lines            13997    14004       +7     
===============================================
  Hits              9279     9279              
- Misses            4718     4725       +7

Impacted Files	Coverage Δ
app/api/attendees.py	37.22% <0%> (-2.01%)	⬇️

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[shreyanshdwivedi]

@mrsaicharan1 @iamareebjamal @uds5501 @kushthedude please review

[Anupam-dagar]

@shreyanshdwivedi looks good to me.

[iamareebjamal]

Looks good otherwise

[shreyanshdwivedi]

@uds5501 @iamareebjamal I've updated the checks. Please review

[kushthedude]

@shreyanshdwivedi , Why did we restrict the orders of a combination of Paid and Free Tickets for unverified users?

[shreyanshdwivedi]

@kushthedude beacuse the issue I mentioned asked to apply check on free tickets - not free orders

[kushthedude]

@shreyanshdwivedi @iamareebjamal Shouldn't we restrict free orders rather than free tickets ?