Export now functionality aka Instant export

I have worked on a couple of functionalities that allows the editor see the synonyms in production instantly, rather than awaiting cron:

• Let export be possible by clicking an "Export now" button
• Trigger search engine to reload in order to load the synonyms

1) Export
I've separated the export functionality from the cron, allowing the export to be done without cron. The administrative page has gotten an "Export now" button that will trigger the export.

2) Reload synonyms
I've added a possibility to make a GET request to a configurable URL after export. For a Solr search engine this allows us to reload the Solr core in order to make Solr read the updated synonyms.

As part of the changes in the export I have incorporated the "It should be possible to disable default cron functionality" → issue.