Manually create todo for issuable

## What does this MR do?

Adds a button to the sidebar in issues & merge requests to allow users to manually create a todo item themselves.

## What are the relevant issue numbers?

Closes #15045 

## Screenshots (if relevant)

![Screen_Shot_2016-06-07_at_09.52.14](/uploads/00af70244c0589d19f241c3e85f3d63d/Screen_Shot_2016-06-07_at_09.52.14.png)

![Screen_Shot_2016-06-07_at_09.52.06](/uploads/e232b02208613a4a50cff4d1e6f119ff/Screen_Shot_2016-06-07_at_09.52.06.png)

![Screen_Shot_2016-06-07_at_09.51.14](/uploads/f1d36435d49ab882538ae2252bec8086/Screen_Shot_2016-06-07_at_09.51.14.png)

See merge request !4502