Add link for searcing issues on issue landing page

[TraceyC77]

https://github.com/getsolus/packages/issues/new/choose

Add link to search the tracker, with verbiage to search for an issue before creating a new one. As the issues list gets bigger, it's more unwieldy for the Solus Team to find if there are duplicate issues. This will help trim down the duplicates.

https://github.com/getsolus/packages/issues https://github.com/getsolus/packages/issues?q=is%3Aissue+is%3Aopen+label%3ABug https://github.com/getsolus/packages/labels/Package%20Request https://github.com/getsolus/packages/labels/Package%20Update%20Request

[TraceyC77]

Related - https://github.com/getsolus/packages/pull/1486

[ermo]

@davidjharder Maybe you can think of a good solution here?