Manually entered permalinks should be validated

[jhung]

Describe the bug

It is possible for someone to publish content with a specified permalink that is invalid. For example, this commit specified a permalink odd which was invalid.

Expected behavior

Some validation should happen upon submission to ensure permalinks are properly formed. i.e. /your-slug/index.html

[jhung]

To validate permalink input, the permalink field will need to be implemented as a component which will require additional time and resources.

For now, improving the labelling and help text might help reduce the occurrence of this error.