[feature] Made admin/submit_line.html extendable

[pandafy]

Changes

#Please explain changes made in your PR#

Screenshots

#For UI changes, please share a screenshot#

Checklist

• I have read the contributing guidelines.
• I have manually tested the proposed changes.
• I have written new test cases to avoid regressions. (if necessary)
• I have updated the documentation. (e.g. README.rst)
• I have added [change!] to commit title to indicate a backward incompatible change. (if required)
• I have checked the links added / modified in the documentation.

#Closes #(issue-number)#

[coveralls]

coverage: 96.18%. remained the same
when pulling 9c0aaa1 on extendable-submit-line
into 1951fad on master.

[nemesifier]

Suggested change

	{% if button.html %}
	{{ button.html }}
	{% if button.raw_html %}
	{{ button.raw_html }}