Validate starting site before beginning the hike

[SylvanSign]

don't want to transport to graph and then have nothing work because they're searching for ''

[SylvanSign]

please also make the following QoL changes to the searchbar:

• make it so only lowercase letters can be typed (maybe css for visuals + final convert to lowercase in JS before sending to server)
  • without this, we might add a node for "foo.com" and "Foo.Com" even though they're equivalent
• more generally, make it so that you can't type spaces or other invalid characters (no valid site for our purposes will have spaces or angle brackets or anything else like that). Should just allow something like [a-z0-9\.]?