Use web prefix for main API

[zhouzhuojie]

Description

Fix #225. Context is that UI's api path can be relative, and we can also prefix our main api with FLAGR_WEB_PREFIX

Motivation and Context

How Has This Been Tested?

Tested locally with and without FLAGR_WEB_PREFIX

FLAGR_WEB_PREFIX='/foo' make run

make run

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)

Although breaking changes to the behavior of FLAGR_WEB_PREFIX, but the scope should be small, since it only applies to deployment after #222

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.

[codecov-io]

Codecov Report

Merging #234 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
- Coverage   83.87%   83.76%   -0.12%     
==========================================
  Files          24       24              
  Lines        1433     1435       +2     
==========================================
  Hits         1202     1202              
- Misses        174      175       +1     
- Partials       57       58       +1

Impacted Files	Coverage Δ
pkg/config/middleware.go	74.8% <0%> (-1.16%)	⬇️

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