added "allow multiple builds" checkboxes

[lprimak]

optionally turn off functionality introduced in #41
for the other 1% of people wanting multiple builds triggered per event

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[bluesliverx]

I think it might be better to implement a check if a build has started for the specific job so that we don't have duplicates. I'll investigate that, but this will work as a backup in case that doesn't pan out. However, I'm really trying to avoid death by checkboxes here, so I'm hesitant to merge it.

[lprimak]

@bluesliverx see this #27 (comment)
I don’t think the queue has to be checked.
I think if the above can be fixed it’s good to go. I think it’s very simple I just don’t have the time to work on it

[lprimak]

superseded by #48