Fixed scroll bar issue in channel list

[nephix]

Thank you for submitting this pull request :)

Fixes #1427

Short description

There's currently another scroll bar issue that is not related to this one: #1439

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)

No steps neccessary, here are GC & FF screenshots:

[codecov]

Codecov Report

Merging #1440 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1440   +/-   ##
=======================================
  Coverage   96.10%   96.10%           
=======================================
  Files         129      129           
  Lines        4801     4801           
  Branches      999      999           
=======================================
  Hits         4614     4614           
  Misses        139      139           
  Partials       48       48           

Flag	Coverage Δ
#dapp	91.58% <ø> (ø)
#sdk	97.79% <ø> (ø)

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

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[kelsos]

lgmt thanks