Refactor application layout (potential BREAKING CHANGE)

[seapagan]

Move the main application code and modules into the app subdirectory of the root. This tidies up things a bit.

However, it may be a **BREAKING CHANGE** for those who have already customized the app.

[codecov]

Codecov Report

Merging #127 (779ace5) into main (14bc2be) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files          22       22           
  Lines         518      518           
=======================================
  Hits          508      508           
  Misses         10       10           

Impacted Files	Coverage Δ
app/config/settings.py	100.00% <ø> (ø)
app/models/enums.py	100.00% <ø> (ø)
app/resources/config_error.py	83.33% <ø> (ø)
app/schemas/email.py	100.00% <ø> (ø)
app/schemas/examples.py	100.00% <ø> (ø)
app/schemas/request/auth.py	100.00% <ø> (ø)
app/schemas/response/auth.py	100.00% <ø> (ø)
app/config/helpers.py	100.00% <100.00%> (ø)
app/config/metadata.py	100.00% <100.00%> (ø)
app/database/db.py	100.00% <100.00%> (ø)
... and 12 more