Move MongoDB database module into core

[jacobtomlinson]

This issue covers adding the MongoDB database module to core.

Background

A while ago we began moving connectors and databases from external plugins into the core of the project (see #185 for more context). We started with slack and websockets connectors but need to go through all the other existing plugins and move them into the core.

Steps

• Make a new submodule directory in opsdroid.database and copy the database code over.
• Update the requirements.txt with any dependencies from the database if necessary.
• Write tests for the database. (See the Slack connector tests for inspiration).
• Copy the relevant information from the database README.md into a new documentation page.
• Add the new page to the mkdocs.yml.
• Add to the list of databases.
• Add a deprecation notice to the old database module. (See the slack connector)

[limberger]

Hi may i work on this issue? I'm new to opensource.

[jacobtomlinson]

Please go ahead!

[limberger]

Tanks!!!