Allow for using a different site name/title in sticky nav

[rnagle]

Some organizations have very long site names which causes the sticky navigation to wrap on smaller screens:

To account for this, we should allow the sticky nav site name to be changed. In the example above, it might be appropriate to use "C-HIT" since the site url is c-hit.org.

For sites that can not shorten their name in this way and do not have the option or resources to produce a suitable logo for this application, we should fallback to using ellipsis to keep the sticky nav from wrapping:

Checklist:

• Add a theme option along the lines of "Alternate site name for use in sticky navigation."
• If the theme option is set, use it instead of the site name ("Site TItle") defined in the Settings > General admin page.
• Add the css to truncate site names with "..." in the event they are too long.