fix: duplicate registration on hackathon onboarding

[walidmujahid]

To prevent duplicate hackathon registrations under an event:

• return error on POST to /register_hackathon/ if user already registerd
• if no poll on hackathon onboard page, disable 'Join Now' button on click
• if poll on hackathon onboard page, disable poll submit button on click

fixes #7070

[codecov]

Codecov Report

Merging #7290 into master will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7290      +/-   ##
==========================================
+ Coverage   26.10%   26.12%   +0.01%     
==========================================
  Files         300      300              
  Lines       29698    29700       +2     
  Branches     4409     4410       +1     
==========================================
+ Hits         7752     7758       +6     
+ Misses      21674    21670       -4     
  Partials      272      272              

Impacted Files	Coverage Δ
app/dashboard/views.py	10.42% <0.00%> (-0.01%)	⬇️
app/dashboard/embed.py	31.60% <0.00%> (+3.44%)	⬆️

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[octavioamu]

@walidmujahid notice you are just adding a disable class not actually disabling the button + if the request fails should "reenable" the button.

[walidmujahid]

Thinking back on how I did this, the click event is still running. I should have probably just set a conditional and exited -and added the class for possibly UX. Sorry for the trouble @octavioamu, I should have noticed the error before it was merged. I will try to get a fix out later today.

[octavioamu]

No problem just add attr disabled I stead of class and remove it on error