keep current episode number in sidebar

[zkamvar]

This will keep the current episode number in the sidebar.

Note that the sidebar mechanism really needs to be overhauled because this is
a bit over-engineered.

This will fix #432

[zkamvar]

I can confirm that this works in the integration test: https://github.com/carpentries/workbench-integration-test/blob/6bb38d9129b117e8b7f77dc5889cf614a52b8dd5/03-coffee.html#L155-L166

[zkamvar]

😭 HADLEY IS BREAKING MY BRITTLE TESTS AGAIN rstudio/renv#1350

── Failure ('test-manage_deps.R:60:3'): manage_deps() will create a renv folder ──
`\.` does not match "knitr".
Actual value: "→ Searching for and installing available dependencies\\n→ Recording changes in lockfile\\n\\n\\nprocessing file: /tmp/RtmpOekirg/working_dir/RtmpnKnfGb/file315b3eb77a60/lesson-example/episodes/introduction\.Rmd\\noutput file: /tmp/RtmpOekirg/working_dir/RtmpnKnfGb/file315b3eb77a60/lesson-example/site/built/introduction\.md\\n"
Backtrace:
    ▆
 1. ├─base::suppressMessages(...) at test-manage_deps.R:60:2
 2. │ └─base::withCallingHandlers(...)
 3. ├─... %>% expect_output("knitr") at test-manage_deps.R:61:4
 4. └─testthat::expect_output(., "knitr") at magrittr/R/pipe.R:136:2
 5.   └─testthat::expect_match(...) at testthat/R/expect-output.R:47:4
 6.     └─testthat:::expect_match_(...) at testthat/R/expectations-matches.R:42:2