Show standard message when dataset list is viewed

[nitrosx]

Summary

When datasets list is selected, do not load any dataset in the main list section of the UI the "apply" button is pressed.

Rather than loading a list of datasets that is not meaningful for the user, scicat should start with a blank table with some text highlighting that the user will see results once they filter or search. SciCat will perform a search on all datasets only if the user specifically say so by submitting a search without conditions.

This issue has been created from backend issue #607