Some images get broken because of the relative path used in Excel/Html export

Summary

When using Excel export, some images are unable to be displayed in Excel (For example, icon for Customer Request Type) because the "src" URL used for the img tag in the Excel file is a relative path and no base tag is defined.

Steps to Reproduce

1. Install JIRA Service Desk and enable Excel export by following the instructions in this KB.
2. Create a new Sample SD project.
3. In the issue navigator, add "Customer Request Type" to display column.
4. Select the issues from the sample project.
5. Verify that the icon of "Customer Request Type" is correctly displayed in JIRA.
6. Export issues in Excel.
7. Open the export file with MS Excel.

Expected Results

icons of "Customer Request Type" are displayed correctly in the Excel file.

Actual Results

icons of "Customer Request Type" are broken.

Notes

Same issue for the HTML Export in JIRA 7.4.0 (EAP-20170613112820) which is currently unreleased.