Add configuration option to set a list of files that should be searchable regardless of their size

Created by: beyang

We currently exclude large files (> 1MB) from searches (https://sourcegraph.com/github.com/sourcegraph/sourcegraph@dd08f0db515f5c89e818afadd5e8af6d9250ca5b/-/blob/cmd/searcher/search/store.go#L280). In most cases, this is preferred, as large files are often generated and users do not want to view them in search results.

In some cases, however, a user would like to increase the max file size limit.

Proposal: add a maxFileSize search query keyword that overrides the default max of 1MB.

Other customer context: https://sourcegraph.slack.com/archives/G9EN3TJDD/p1550603834001000