[UBP for individuals] Configure a usage limit for usage-based billing in personal settings

[jldec]

The design and behavior of this UI should be very similar to the UBP usage limit input for teams.

• The limit value is pre-set to x credits, where x = the free credit allowance for individuals.
• Thie UI should allow the value to be set above (or equal to) the free credit allowance, but not below.

[andrew-farries]

Removing my assignment from this now that #12942 is merged.

Users can now sign up for UBP, so this issue is unblocked.

[AlexTugarev]

This needs to be verified. I think it's already solved.

[AlexTugarev]

Verified in a test deployment, cf. #13076 (comment)
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