feat: Add a limit for the number of MyInfo fields that can be added

[frankchn]

Problem

MyInfo fields must be limited to 30 per form on Email mode to comply with PII guidelines. In exchange, we get access to all fields.

Closes #1305

Solution

Adds limits and validators to the appropriate places to limit MyInfo fields to 30.

Screenshots

Tests

• Add 30 MyInfo fields to a form.
• Check that under the build tab, MyInfo fields can no longer be added
• Check that MyInfo fields cannot be duplicated
• Delete a Myinfo field (such that the form now has <30) and check that you can now add and duplicate Myinfo fields
• Manually add an additional MyInfo field to the form by editing the database. Check that when you try to edit the form subsequently, you get an error message saying to check that the form has max 30 MyInfo fields.
• Run the following query to ensure that there are no existing forms with >30 MyInfo fields:

db.getMongo().setReadPref('secondary')
db.getCollection('forms').aggregate([
    { $match: { 'form_fields.myInfo': { $exists: true } } },
    { $unwind: '$form_fields' },
    { $match: { 'form_fields.myInfo': { $exists: true } } },
    { $group: { _id: '$_id', count: { $sum: 1 } } },
    { $match: { count: { $gt: 30 } } }
])

• After release has been deployed, run the above query again to check that there are still no forms with >30 MyInfo fields. If there are, the form admin must be contacted.

[mantariksh]

thanks @frankchn! this looks good, but I'll need to write and run a database query first to check that there aren't already forms with >30 MyInfo fields. if the current database state checks out then I'll approve and merge this. pending last comment below

[liangyuanruo]

thanks @frankchn! this looks good, but I'll need to write and run a database query first to check that there aren't already forms with >30 MyInfo fields. if the current database state checks out then I'll approve and merge this.

Update on behalf on @mantariksh - there aren't any such forms, so the PR is safe.

[mantariksh]

one remaining bug, otherwise looks good!