Audit avatar sizing assumptions

[joshtrichards]

See nextcloud/files-clients#18

Since v24.0.0 the Server Avatar API only returns 64 or 512.

When other sizes are requested, the server API "rounds" to the nearest match unbeknownst to the requesting client.

Concerns:

• When the client retrieves the wrong size, caching is impacted (due to mismatched URLs)
• When the client receives the wrong size, there may be display or other cosmetic issues.

Needs:

• Only 64 or 512 should be retrieved to avoid breaking caching.
• Any assumptions about sizes other than 64/512 being available should be handled.

References:

• Only allow avatars in 64 and 512 pixel size server#31010
• fix(core): Limit valid avatar sizes server#47211