Support for additional permalink structures

[ErisDS]

Currently, Ghost supports the following permalink structures:

• Standard: /:slug/
• Dated: /:year/:month/:day/:slug/

In order to make it easier for people to move their blogs from other platforms to Ghost, we want to support any permalink structure which contains any of the following items:

• :slug
• :id
• :year
• :month
• :day
• santised custom text containing a-z, 0-9, - or _

To be a valid permalink, each item must be separated by a '/', and there should be a maximum of 5 items. the permalink must contain one of :slug or :id so that the post can be identified.

So I could set my blog posts to have a URL like any of these:

• `/:year/:month/:slug/
• `/:year/:slug/
• `/blogpost/:id/
• /post/:year/:month/:day/:slug/

At the completion of this issue, Ghost should support these structures if the permalink setting is modified directly in the DB. The UI for configuring the permalink in admin is covered by issue #1631.

Note:

Some things we should consider for the future are:

• whether or not to support adding .html to the end of URLs so that imports from blogger work,
• whether to import custom data and allow that in the URL, like the id which is in tumblr URLs

We do want to make it possible to have an app that does mass re-writing/redirecting of URLs... so we could solve these problems by having an app 301 redirect the .html out of blogger urls, and the id out of tumblr urls, in which case blogger becomes /:year/:month/:slug/ and blogger becomes /post/:slug/or even the default /:slug/.