Rework Nomadnet Links - Don't establish a new link for each request

[liamcottle]

Now that #20 has been merged in, and form fields are now supported, I will need to look into reworking how the internal RNS links for nomad network page downloads are handled in MeshChat, as some sites are using the link id to determine if a user is logged in.

However, since MeshChat establishes a new link for every page that loads, the link id changes, which breaks that functionality.

For example, the etux node app template files:
https://codeberg.org/etux/Node_App_Template/src/branch/main/login.mu#L23
f166946957ffb27d92b196df868cc20c:/page/index.mu

Should probably keep an in memory cache of established links to nodes, and use that if possible, otherwise if none exists, or it's stale/closed, establish a new link...

[liamcottle]

This will do the trick for now: bd3c6fd