Project management automation: Fix 'add milestone'

[noisysocks]

Octokit returns all resources wrapped in a data key, e.g. data.milestone instead of milestone. Also, octokit.repos.getContents() returns a base64 encoded string.

(I'm a big idiot who didn't read https://octokit.github.io/rest.js/#usage!)

Confirmed this by using my local Node REPL:

$ node
> let { GitHub } = require( '@actions/github' )
> let octokit = new GitHub( 'INSERT_PERSONAL_ACCESS_TOKEN_HERE' )
> octokit.issues.get( { owner: 'WordPress', repo: 'gutenberg', issue_number: 17156 } ).then( r => result = r )
> result.data.milestone
null
> octokit.repos.getContents( { owner: 'WordPress', repo: 'gutenberg', path: 'package.json' } ).then( r => result = r )
> JSON.parse( Buffer.from( result.data.content, result.data.encoding ).toString() ).version
'6.3.0'
> octokit.issues.listMilestonesForRepo( { owner: 'WordPress', repo: 'gutenberg' } ).then( r => result = r )
> result.data[0].title
'Future'