fix: improve timelock ux

[gabaldon]

This pr implements the suggestions from discussion #1701.
The element-UI locale updates itself when the language changes and the date-picker input is hidden now behind the advance options button.

[aesedepece]

Can we use ▲ and ▼ or ▴ and ▾ on the "Advanced options" label? I think it helps understanding that it can be toggled on / off.

[aesedepece]

Trying to think of similar UIs, I tend to think that it is customary to place the advanced options after the "Advanced options" toggle label, so it feels that those options are somehow "contained" inside a section that was initially collapsed. In other words, when I click "Advanced options", I'm always expecting the options to appear down below.

[aesedepece]

The "Now" option inside the date picker seems redundant to me.

If we can't remove it, I'd rename it to "No timelock" (of course, using the current date as timelock will equal no timelock in practice).

As the timelock is an optional feature and has two very clearly differentiated valid values (no date vs. some date), I'd also appreciate a quick way to clear this field. Something like an x inside the text box or next to it would work for me.