Add GC pre-screening before TC diligence for donations

[dyladan]

This update follows discussion in the GC meeting about the donation process. In the meeting it was agreed that the GC should prescreen donation proposals before the TC is required to conduct due diligence and consider alternatives in order to reduce the workload on the TC.

[mtwo]

This was widely liked on a GC + TC Slack conversation, but we should have more TC people formally weigh in before merging