Build ingest 413 messages from the byte-limit constants #466

@renato2099

From the review of PR 437 (comment):

this string should ideally use variable we defined, otherwise it is easy that they go out of sync

tagbase_server/tagbase_server/upload_stream.py defines SYNC_MAX_BYTES (100 MiB) and JOB_MAX_BYTES (2 GiB), overridable by TAGBASE_SYNC_MAX_BYTES and TAGBASE_JOB_MAX_BYTES. The 413 details still say "at most 100 MiB" and "at most 2 GiB". Format those sentences from the active limit.