Added config for base path for web UI

[SebastianOsuna]

Description

Added FLAGR_WEB_PREFIX env so it is possible to serve the static web UI from another path different to /

Motivation and Context

For example, if deploying flagger in ECS behind and ELB it wouldn't be possible to deploy Flagr anywhere but in the /* listener since the Web UI is served in / and the API in /api/v1, this would allow to server the UI from /web/flagr/* for example.

How Has This Been Tested?

Built the code in a docker image, set the environment variable to /web/flagr and the UI was loading from that prefix

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [?] My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• [?] All new and existing tests passed.

[SebastianOsuna]

I'm unsure if I am following the code style or if this change needs a test, please let me know, or any other comments. Thanks

[codecov-io]

Codecov Report

Merging #170 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage      93%   93.02%   +0.02%     
==========================================
  Files          25       25              
  Lines        1330     1334       +4     
==========================================
+ Hits         1237     1241       +4     
  Misses         63       63              
  Partials       30       30

Impacted Files	Coverage Δ
pkg/config/middleware.go	100% <100%> (ø)	⬆️

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[zhouzhuojie]

lgtm, thanks!