Accept or Obsolete "Provisional" OEPs

[sarina]

Several OEPs were authored aspirationally, and then never implemented. Other were partially implemented, but have stalled or diverged from reality. Yet others should be followed by the community, but are erroneously marked as "Provisional".

In order to establish OEPs as a trustworthy source of documentation, we should mark OEPs that never came to be as "Replaced" or "Obsolete" (whichever or is more appropriate). We should revise and "Accept" OEPs that are still relevant but mismatched with reality. We should simply "Accept" OEPs that are for all practical purposes adopted by the community.

Sub-issues have been made for all the found Provisional OEPs & ADRs.

[bmtcril]

I'll refresh my memory on OEP-30 and try to tackle that first. I think I'd like to get community feedback on how we'd like to proceed with it before taking any action to obsolete it for sure.

OEP-26 should be accepted, but may need a refresh. I'll try to review this week as well.

[msegado]

I believe OEP-15 was implemented by @ha-D in openedx/edx-platform#28411, and the OEP was updated by @xitij2000 in #396 to match the deployed implementation. I'm not sure if things have drifted since then, though?