Fixes scrollbar displaying in channels screen without many items

[kelsos]

Fixes #1490

Short description
Makes sure that scrollbar is not shown unless there are enough items to need a scrollbar

Definition of Done

• Steps to manually test the change have been documented
• Acceptance criteria are met
• Change has been manually tested by the reviewer (dApp)

Steps to manually test the change (dApp)

1. Go to channels.
2. If you have one or two channels, no scrollbar should display.

[codecov]

Codecov Report

Merging #1502 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1502   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files         140      140           
  Lines        5000     5000           
  Branches      927      984   +57     
=======================================
  Hits         4803     4803           
  Misses        153      153           
  Partials       44       44           

Flag	Coverage Δ
#dapp	91.93% <ø> (ø)
#sdk	97.64% <ø> (ø)

Impacted Files	Coverage Δ
raiden-dapp/src/components/navigation/Channels.vue	100.00% <ø> (ø)

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[nephix]

@kelsos with how many channels have you tested this because there was a bug #1427

[kelsos]

I had to open 3 channels otherwise the scrollbar would not appear, but there is an empty space following the last expanded area so we should still be safe.

[nephix]

Only need to update change log I guess