Cleanup and validate emails on push recipients

What does this MR do?

Validates max number of recipients: It introduces a limit for a max number of recipients. For now the limit is 750. If the number of addresses is higher than the limit, it shows an error message. (See #268207 (comment 505540508) for context of why 750)

Normalizes the recipients list on save: In order to count the recipients we need to clean up the list. For example Andy Soiron asoiron@example.com should only count 1 email address even though it has 3 words. Duplicates are removed from the list in a case-insensitive way. For context why case-insensitive see !55588 (comment 523381019) and !55588 (comment 527103994)

Here is how the form will look like including the error message:

Related issue: #268207 (closed) the issue is confidential, but it was decided that this doesn't need a security MR.

Technical Writing

New texts:

• Error message: "Recipients max number is 750". This follows a pattern that the first word is the form field name.
• Help text for the recipients field: "Invalid email addresses and duplicates will be removed."

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done