Fix URL encoding for community names in notification links

[xenacode-art]

Closes #119

Problem I noticed

In-app notification link fields were building URL paths using raw
community.name values. Community names containing special characters
such as + or spaces produced malformed paths — e.g.
/community/GSoC+2026/requests — which browsers misinterpret,
causing 404s when users click through from their notification panel.

Fix I Implemented

Applied urllib.parse.quote(community.name, safe='') to every
notification link that embeds a community name in the path. This
matches the encoding already used in send_emails.py for email links.

Files changed:

• communities/api_join.py — join request received + approved notifications
• communities/articles_api.py — article submitted notification
• articles/api.py — article submitted notification (community context)

Testing

# Community name with special chars
community.name = "GSoC+2026"

# Before
link = f"/community/{community.name}/requests"
# → /community/GSoC+2026/requests  ← broken

# After
link = f"/community/{quote(community.name, safe='')}/requests"
# → /community/GSoC%2B2026/requests  ← correct

[armanalam03]

@xenacode-art There have been couple of changes in the test branch regarding Notifications. Would request you to sync your changes. Before raising a new PR, please check again if this issue is re-occurring on https://test.scicommons.org/.