Allow install of other plugins

[nataliecarey]

What

Add the option for users to install plugins that aren't in our list. They should be able to install from:

• npm
• local file system
• github

This can be achieved by having a text input that gets passed straight onto the npm install command, therefore we can just prefix the user's input with npm install. The only check we need to do is to make sure there's no spaces so they're only installing one dependency at a time (to allow our checks to work as usual).

Our usual install process with dependency checking should be followed.

Why

This will help users who want to install plugins we haven't added to our list, it will also help contributors who are trying to install pre-releases as they will be able to see the process users will go through once it's released.

Who needs to work on this

• Designer
• Developer

Who needs to review this

• Designer
• Developer

Done when

• [ ] Users can install plugins which aren't in our list via the UI

[joelanman]

Designs for this:

On the Find Plugins page, add a link to the bottom of the list:

The page itself:

Figma file:

https://www.figma.com/file/sWnqT4u4uup91TuTe9T3tp/GOV.UK-Prototype-Kit?type=design&node-id=321-2&mode=design&t=UGH1LeuuhIUEpQnf-0