Add limit to number of repos resolved from search query for 1-click insight creation

Created by: felixfbecker

It's currently possible to create an insight with a massive list of repos by clicking "Create insight" from a search query that includes all repos on an instance or in an org (which is the default for search queries on premise). This creates insights that we can't support from a scale perspective, and can potentially DDoS the own instance even just from the live preview (it gets worse when it gets added to the dashboard with all the other insights).

We should apply some limit on it (e.g. 20).