Add a begging modal

[iHiD]

I've started this but it needs you to properly do it.

• This should be an on-demand modal that I can call whenever (the "calling whenever" bit is done but it needs to be an actual modal)
• Clicking the negative button (Currently "continue without donating") should close the modal.
• It needs to work in dark theme (does the mentoring flow one currently also look a little unreadable with the blue in dark theme - see images below?)
• What do I need to pass into this from the Ruby helper to make it work?

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