[FEATURE] Support inviting thousands of round members

[aerugo]

At the moment the platform does not comfortably manage more than about 150 users per round.

• Pasting comma separated emails into the invite box does not work for more than about 150 at a time.
• Adding and/or setting balance for is unreliable for more than a few hundred users and usually times out. (addressed separately in issue [FEATURE] Support allocations to thousands of round members #1067)

At the moment, rounds with thousands of users are managed with custom scripts. In practice, Cobudget currently does not support rounds with more than 150 users without custom scripts and manual workarounds.

We need to allow handling large rounds with thousands of users. At the same time, large rounds create very large volumes of emails. It is unwise to allow this on a free instance as it can easily be exploited.

I propose that we do the following to address the invitation issues:

• Limit the number of round members (and pending invites) in free rounds to 100, and allow us to manually override this per round by setting a flag in the database.
• Limit the number of participants in rounds created in paid groups to 1000.
• Limit the number of emails that can be invited through the invite box modal to 100.
• Create a new invitation feature for large invite lists that allows an admin to import a CSV of emails and the starting balance they want for each individual member. This should be tested to make sure that we can confidently send up to 10,000 invites through Postmark.

[alinauroz]

This can be divided into 4 sub-tasks

• Optimize inviteRoundMembers mutation (decrease execution time so request does not timeout)
• Limits (for free/paid accounts + manually set limit through database)
• Testing Bulk emails
• Invite using CSV