Make the 5mb limit on indexing content configurable

Currently the 5mb limit that we place on indexing content is hardcoded.

We should make this configurable. (As per customer request at https://support.atlassian.com/browse/FSH-1407)

However I still think we need to set some sort of sensible upper limit, for example should we allow users to specify 100mb? Can the browser even render a file that large?
We should keep the default at 5mb, and place a warning in the administration about the consequences of raising this value.