Implements tpo/metrics/website#40001.

As part of #34023, we're going to give up on making 50 KiB and 1 MiB
downloads in the near future. In order to continue plotting time to
download these file sizes we're including partial downloads of larger
file sizes.

Implements #30611.

In commit 7af8f108 we started ignoring v3 onion OnionPerf measurements,
because including a third option in addition to "public" and "onion"
would have required writing more code.

However, an analysis in #33434 revealed that v3 onion measurements are
basically no different than v2 onion measurements. As a result we're
turning off v2 onion measurements and combining v2 and v3 onion
measurements in existing graphs.

Implements #34032.

Implements #34109.

The unit_format function in our somewhat outdated scales package
rounds labels to whichever we accuracy we ask for. However, in some
cases this is difficult to do right (e.g., Time to download files over
Tor graph with measurements apparently getting faster over time), in
other cases it's impossible (Advertised bandwidth distribution graph
with 1st and 99th percentile having different orders of magnitude).

The new custom_unit_format function does not round labels and instead
determines more reasonably how many digits it needs to print.

Fixes #34103.

Fixes #33933.

Estimate bridge users by country based on requests by country, if
available, to get more accurate numbers than those obtained from
unique IP address counts.

Fixes #18167.

Estimate relay users by country based on responses to directory
requests to reduce the overall effect of binning and to make relay and
bridge user estimates more comparable.

Implements #18203.

Fixes #32683.

 - style: Lines should not be more than 80 characters.
 - style: Only use double-quotes.
 - style: Put spaces around all infix operators.
 - style: Trailing blank lines are superfluous.
 - warning: local variable ‘...’ assigned but may not be used

Also leave a comment to run lintr after making non-trivial changes.

Still part of #33549.

Implements #24941.

Part of these changes have already been made in earlier commits as
part of #24532. This commit makes remaining changes as part of #33655.

Implements #33549.

Remaining part of #33008.

The new icon is based on clock, solid style in Font Awesome Free
5.21.1. It was converted using Inkscape 0.92.4 using the command:
`inkscape -z -e clock.png -w 16 -h 16 clock.svg`.

Fixes #33453.

Fix provided by RaBe.

Fixes #28317.

 - Handle missing dates correctly.

Implements #33065.

The old PL/pgSQL version of this function made three lookups in the
bwhist table to 1) check whether a row already exists, 2) insert or
update the row, and 3) update the row once again with array sums. The
new SQL version uses the INSERT ON CONFLICT statement introduced in
PostgreSQL 9.5 (Debian stretch has 9.6, buster has 11). The
performance gain measured using metrics-test is impressive, computed
aggregates are equivalent.

Implements #33065.

Fixes #33335.

Fixes #24532.