[dex integration] Define strict layout breakpoints.

[amass01]

As part of integrating dcrdex into decrediton we will need to match both gui's layout breakpoints.

Currently on the dex side we are using bootstrap's famous breakpoints - as mentioned on decred/dcrdex#997 (comment).
I found out that we use way too many layout breakpoints which might it make harder to integrate dex's gui in decrediton:

• 1919
• 1680<x<1919
• 1440<x<1679
• 1679
• 1539
• 1180
• 1179
• 1140
• 1106
• 978
• 955
• 916
• 890
• 800
• 768
• 765
• 746
• 710
• 676
• 670
• 600
• 588
• 541
• 540
• 511
• 500
• 430
• 420 :)
• 409
• 375
• 299

Many of these are used 1-2 times and very close to other breakpoints so we have to improve these and define stricter set of breakpoints and ensure we use the same values all over the place.

[bgptr]

on this

[amass01]

thanks @bgptr 💪🏽
it would be great, if we follow pi-ui & pigui breakpoints here, as we use it as our ui components lib.