Add PhoneField. Add migration helpers from Linkable

[chrispenny]

• Linkable had a concept of a "Phone" link, which didn't currently exist in LinkField. So I've added that. It's really just a duplicate of EmailField, but it returns a tel instead of a mailto.
• Added some instructions based on what I found from migrating my own project. These steps may not be completely comprehensive, but I've tried to at least indicate areas that might need attention.
• Supports Versioned and un-Versioned. However, we expect the Versioned state between legacy (Linkable) and LinkField to match.

[GuySartorelli]

The migration stuff feels quite separate to the phone link functionality. Might be worth splitting it up into two PRs?

[chrispenny]

@GuySartorelli yea, you're right, it's just that the Phone PR would be a blocker for the migration stuff.

I figured this way, if anyone is looking in, needing to perform a migration soon, then they could choose to run this branch as a fork and have everything they need.

[mfendeksilverstripe]

Awesome work 👍 Code is looking good, documentation is quite detailed and it works for me locally. I managed to migrate a bunch of links from my fixture.

I added some very minor changes:

• WS fixes
• added a few doc-blocks
• renamed a few variables (avoid type in variable name)
• simplified a few DB method calls
• opened up some properties / methods to be protected (more flexibility in case someone needs to subclass)