Full text search

> *please mark as* [`feature`](https://github.com/hackmdio/hackmd/labels/feature) request

As a logged in user, I am presented with recently opened pads when visiting the splash page. These are custom to the user client and stored in local storage, as far as I know. There is no known synchronisation with the HackMD server.

This issue tries to anticipate what would be needed to implement full text search over pads.

Given the introductory assumptions, a search service would allow for POSTing a list of pads to crawl, index and query by the frontend client, which is the only source of lists of pads available.