Clear selected project

[NealHumphrey]

• Allow the user to clear the selected project so that none is selected (using 'x' in the upper right?)
• Automatically clear the selected project if the user selects a new filter and the selected project is no longer part of the filtered project list.

[NealHumphrey]

Note that currently, in the project preview partial the initial view has text to the effect of 'No project selected.' However, when the first project is selected (by clicking on it on the map), that text is overwritten with a project preview with no way to get it back. We will want to have the code that adds the project preview add the 'no project' text.

[NealHumphrey]

Also make sure that URL encoding behaves properly and removes the selected project from the url. See this PR for details: #525

[ostermanj]

Note from discussion at Housing Insights meeting:
Clear selected building when it does not match the latest filter, but change the text of the preview pane from "There is no project selected" to something indicating what the previous selection was, with option to reselect it.