[admin][UBP] Grant free credits to users in admin dashboard

[jldec]

Admins should be able to grant additional credits to users E.g. to unblock their usage if they had out-of-control prebuilds before switching to UBP.

Example here (internal)

[jldec]

A reasonable option would be to reset the clock on their free tier so that they start with a fresh 500 credits.

[svenefftinge]

We should allow admins to do these things:

• Add usage entries to adjust the current credit balance in both directions.
  • they need a special usage kind as well as a mandatory description and userId of the admin.
• Adjust spending limit of cost-center
• See usage

@jldec What is the order in which you want to see this in production?
@gtsiolis We need design input on how to switch between different admin areas per team detail (currently we have one page where we list the members).

[jldec]

@svenefftinge priorities from product side are:

1. unblock users who hit their limit due to unforeseen reasons like runaway prebuilds - this is probably best done by adjusting the credit balance, but could also be done with a temporary usage limit bump.
2. grant a permanent increase/decrease in free tier usage limit e.g. for open source maintainers.
3. see current total usage, credit limit, next reset date
4. see detailed usage (workspace sessions)

[svenefftinge]

Unassigning myself, because we need input from @gitpod-io/product-design

[svenefftinge]

Also see #14367

[jldec]

From discussion with @gtsiolis

• Action uses standard hypertext link in admin view to grant extra credits
• Trigger modal form with action/cancel buttons
• Requires a "reason" to be entered by the admin
• Free tier, individual plans only
• One time grant only - entered into ledger
• Enter amount (typically 100 or 200)
• Effect will be to reduce "Credits Used" (number should decrease after)
• nice to have: don't allow granting more than current usage (avoid negative result)
• future enhancement: show extra credit grant in the billing or usage view

Alternative approach (if easier):
Instead of subtracting from credits used, add to usage limit temporarily (reset at the end of the period)

NOTE: This credit grant mechanism may be suboptimal if the period is about to be reset, since unused free credits expire at the end of each cycle when the balance used goes back to 0.