Investigate and resolve code base TODOs

$ grep -rn "TODO" --exclude-dir=lib .
./cmake/build/options.cmake:7:# TODO: actually use these
./src/components/command_line.hpp:72:        // TODO: strip code for valid_groups? (We have no use for it).
./src/components/downloader.cpp:117:                    /* TODO: test this output */
./src/components/downloader.cpp:133:                        /* TODO: warn the user about file removal (or just warn that the file
./src/core/plugin_handler.cpp:107:     * TODO: colour this output to match that of the log screen.
./src/core/plugin_handler.cpp:233:     * TODO: clean this up
./src/core/bindings/CMakeLists.txt:10:# TODO: replace this with FindPython3 when it's released
./src/core/bindings/python.cpp:87:            /* TODO: Don't create a new wrapper instance every time */
./src/core/plugin_handler.hpp:113:         * TODO [doc] return const and make const
./src/tui/screens/multiselect_menu.cpp:28:         * TODO: Remedy this.
./src/tui/screens/multiselect_menu.cpp:155:         * TODO: fix this for cases where the cursor has
./src/plugins/libgen.py:12:# TODO list:
./src/plugins/libgen.py:18:#   - Fix all TODO:s
./src/plugins/libgen.py:193:        #         'v': non_empty(e.volume, e.year),  # TODO: e.volume should be a string?
./src/plugins/libgen.py:283:        # TODO: check if the same HTML is given when using gen.lib.rus.ec host.
./src/plugins/libgen.py:362:                # TODO: process bookfi and B-Ok?
./src/runes.hpp:37:    // TODO: drop str_const
./tests/CMakeLists.txt:81:    # TODO: add one for the detach test
./CONTRIBUTING.md:9:If you're looking for some smaller issues to fix, I've made sure to sprinkle `TODO: ...`s throughout the code base.
./CONTRIBUTING.md:14:$ grep -rn "TODO" --exclude-dir=lib .
./etc/default.nix:7:  # TODO: PR these to nixpkgs
tmplt / bookwyrm

Investigate and resolve code base TODOs #61
