Fix broken links and anchors. Also add MIG definitions for GUI and Is…

[rosiel]

…landora Model.

Purpose / why

MIG worked on definitions for the glossary, for GUI and Islandora Model.

WHen I went to add them, a large number of links and anchors were broken.

What changes were made?

Add definitions for GUI and Islandora Model to the glossary
Fix broken links.

Verification

Interested Parties

• @Islandora/documentation
• @Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?