validate path before saving

[akash1810]

We've seen some paths being created with whitespace, which isn't great, so explicitly reject invalid paths.

A valid path is one that's a hyphenated alphanumeric.

An invalid path:

• doesn't start with an alphanumeric
• contains white space
• contains single quotes

Failure will cause flexible-api to return a 409 with response invalid-identifier, which is already handled by Composer here.

Its easier to view the diff without whitespace - https://github.com/guardian/path-manager/pull/23/files?w=1

[akash1810]

cc @tomrf1