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.

Start by tracing how the splash page obtains recently opened pads and how that list is stored in local storage. Then determine the frontend client’s requirements for submitting pads to a search service and receiving indexed queries. Done means the scope and interface for searching the available pad list are defined.

索引モデルが issue の本文から書いたものです。