[api] da-admin POST /source/ should do a size check

[mhaack]

With Helix limiting the file size of pages, images, PDFs etc. da-admin should follow the same rules and reject POST requests if they exceed the size limits.

Currently the following limits are enforced by the EDS Admin API:

• page / document size limit: 1MByte
• max image size limit 1MByte

Limits on sheet sizes or file sizes are currently documented (https://www.aem.live/docs/limits#file-size-limits) but not enforced.