site: do not get icon repeatedly

[chappjc]

A variety of notifications from the client backend cause the frontend to update an asset's "button" on the wallet's page. In part, this involves setting tmpl.img.src, which unfortunately initiates a GET request for the resource, whether the path has changed or not.

For instance, my testnet bch wallet was spewing wallet state notes, causing repeated fetches of the bch.png icon each time:

If the browser isn't sharp about caching these resources, it hits the backend repeatedly:

2023/02/23 13:54:17 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 12.844756ms
2023/02/23 13:54:20 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 12.682972ms
2023/02/23 13:54:23 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 13.716873ms
2023/02/23 13:54:26 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 12.674457ms
2023/02/23 13:54:29 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 11.653743ms
2023/02/23 13:54:32 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 8.95034ms
2023/02/23 13:54:35 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 9.302125ms
2023/02/23 13:54:38 "GET http://127.0.0.2:5758/img/coins/bch.png HTTP/1.1" from 127.0.0.1:54706 - 200 4103B in 14.660306ms

This PR updates the updateAssetButton method of WalletsPage so that it does not re-assign the img.src field if it has already been set. This method is called by at least 4 notification handlers, so it is the main offender.

[chappjc]

I first did this with the nullish coalescing assignment operator (??=) but to my surprise the src attribute is never null for an img tag, even if it's not there in the html (as with our template); it's just empty, so we have to go with the falsy condition.

[buck54321]

We could also leverage the Expires or Cache-Control headers for images, so even if a request comes through, the response will be tiny.

[chappjc]

We could also leverage the Expires or Cache-Control headers for images, so even if a request comes through, the response will be tiny.

Yup, I had a second commit I was going to push with this PR, but just this change seemed to do the trick (when the browser doesn't have the "Disable cache" box checked :embarrased:).

3769c4c