Add flower to dev dependencies

[MisRob]

Description

Add Flower to dev dependencies and mention that we have it available in README.

@aronasorman When I run pip-compile, it also changed other dependencies' format (maybe it's because I am using the newest version?) Is it okay? To make the review easier, I run pip-compile on the original requirements-dev.in without adding any new updates and committed the diff. Then I added the flower package to requirements, recompiled them, and committed this change in another commit.

Issue Addressed

I hope this will make locating this tool easier for developers who are not so familiar with backend tooling like me. Also, I noticed that we actually run flower already as part of services command so I assumed that it would work, and it took me a while to figure out that it wasn't working just due to a missing dependency.

Steps to Test

• Run pip install -r requirements-dev.txt
• Run yarn run services
• See Flower dashboard on http://localhost:5555

[MisRob]

I specified this version because when I didn't do it, I was getting this error during compilation:

Tried: 0.0.1, 0.0.2, 0.0.3, 0.0.4, 0.0.5, 0.0.6, 0.0.7, 0.0.8, 0.0.9, 0.0.10, 0.0.11, 0.0.12, 0.0.13, 0.0.14, 0.0.15, 0.0.16, 0.0.17, 0.0.18, 0.0.19, 0.0.20, 0.0.21, 0.1.0, 0.1.1, 0.2.0, 0.3.0, 0.3.1, 0.4.0, 0.4.1, 0.4.2, 0.5.0, 0.6.0, 0.7.0, 0.7.1, 0.8.0, 0.8.0, 0.9.0, 0.9.0
There are incompatible versions in the resolved dependencies:
prometheus-client==0.7.1 (from -c requirements.txt (line 66))
prometheus-client==0.8.0 (from flower==0.9.7->-r requirements-dev.in (line 42))

I didn't want to touch production dependencies in this PR. Flower 0.9.4 is the newest version that worked well with our current dependencies. Is this approach okay?

[rtibbles]

Absolutely.

[MisRob]

Frontend linting check failure is fixed in #2957

[codecov]

Codecov Report

Merging #2958 (f0a9e26) into develop (6cd38c6) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## develop #2958 +/- ##
========================================
Coverage 85.53% 85.53% ========================================
Files 299 299 Lines 15827 15827 ========================================
Hits 13538 13538 Misses 2289 2289 

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