Allow configurable max blob size for indexer

Problem to solve

GitLab won't index files larger than 1MiB.

Further details

Some GitLab users may wish to use the Elasticsearch integration with larger files. If a user is hosting their own ES server then it should be capable of indexing/searching larger files. Currently, we have a max blob size of 1MiB hard-coded into the ruby indexer and the newer go indexer.

I did test and confirm that the default search in GitLab(without ES) can search through and display results for text files that are 6+ MB in size.

Proposal

The max blob size should be a configurable setting in the indexer(s).