Refactor API calls to support self-hosted on Jira connect app

What does this MR do and why?

Note: This MR is split from the original changes in !89378 (closed).

This MR refactors all existing API calls using axios to support calls to a self-hosted instance (from within the context of GitLab.com). This works by creating a separate axiosInstance from the one we usually use and setting its default baseURL with this code (to be added in a later MR):

export const setApiBaseURL = (baseURL) => {
  axiosInstance.defaults.baseURL = baseURL;
};

Screenshots or screen recordings

No changes to existing functionality.

How to set up and validate locally

See the original MR description for video run-through and detailed steps: !89378 (diffs)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #356361 (closed)