Relayer UI improvements

[isSerge]

• Currently, we have 40+ chains and the list of chains is sorted on every new Subspace block (based on the block height). It seems like not a great UX because list items jump all the time
• Last updated column shows inaccurate values - it starts a timer when the page was loaded, but the actual value can be larger if the relayer (backend) did not submit any blocks for a long time. Instead, we can show the time since the last extrinsic was submitted by the relevant account