Escape nicknames in patrick responses.

[JoBeGaming]

See https://discord.com/channels/116914772766752769/165640273810948097/1544019167798239233 for embeds, and https://discord.com/channels/116914772766752769/1216162486152335471/1544010187546038282 as well as https://discord.com/channels/116914772766752769/1216162486152335471/1544010104511266841 for nickame usage in normal commands.

For tests, the output of the test attached was sent in https://discord.com/channels/116914772766752769/165640273810948097/1544010979694608516, and seems to raise no issues with Discord Markdown. See https://discord.com/channels/116914772766752769/165640273810948097/1544023339042087113 for tests related to links. Note that I've not implemented direct link escaping (https://www.youtube.com) yet.

assert escape_username("> job") == r"\> job"
assert escape_username("*job*") == r"\*job\*"
assert escape_username("**job**") == r"\*\*job\*\*"
assert escape_username("_job_") == r"\_job\_"
assert escape_username("__job__") == r"\_\_job\_\_"
assert escape_username("`job`") == r"\`job\`"
assert escape_username("> job") == r"\> job"
assert escape_username("~~job~~") == r"\~\~job\~\~"
assert escape_username("||job||") == r"\|\|job\|\|"